The Senate gave unexpected approval yesterday to President Reagan's proposed nuclear trade agreement with China, paving the way for U.S. firms to begin exports to Peking after Dec. 11.

Passage by voice vote during a break in debate on the farm bill surprised some leading critics, who were not present. The verdict follows committee action last week attaching conditions drafted by a leading critic of the measure, Sen. Alan Cranston (D-Calif.)

But other critics, who joined Cranston in arguing that the original agreement did not provide adequate safeguards against the spread of nuclear weapons, maintain that the new conditions are insufficient.

Sen. John Glenn (D-Ohio) was among the latter, calling the pact a "bold and alarming step backwards in U.S. support for the international nonproliferation regime."

The 30-year pact, first initialed during President Reagan's trip to Peking in April 1984, establishes the legal framework to allow the U.S. nuclear industry to bid for a share of China's estimated $6 billion nuclear power plant construction program.The House is expected to approve an identical version of the agreement soon.

In a speech after the vote, Glenn said that he had not tried to amend the measure further because extending debate beyond Dec. 11 would have caused the original version to take effect automatically.

"I certainly prefer to have the compromise resolution, flawed as it is, than to have nothing," Glenn said.

Cranston contended that his changes provide "very effective safeguards without requiring any renegotiation of the agreement with China." He noted that the Foreign Relations Committee vote last week was "a pretty solid" 14 to 3.

"You'll never get a perfect agreement, but I think we've got a pretty good one," he said.

In a statement, the State Department agreed, saying: "This resolution in no way undercuts the implementation of our agreement with China." It promised to "continue to develop a dialogue with China on nonproliferation policy and nuclear cooperation" and a dialogue with Congress on U.S.-China relations.

The pact was controversial from the outset because of China's previous endorsement of nuclear proliferation, its unwillingness to submit to International Atomic Energy Agency (IAEA) safeguards, and intelligence reports that Chinese technicians had been helping at a suspected Pakistani bomb-development site.

Administration officials sought additional Chinese assurances. Although these were verbal, the pact was submitted to Congress during the visit here of Chinese President Li Xiannian last July.

After the Nuclear Regulatory Commission and others raised concerns, Glenn proposed amendments that would have required China to clarify its nonproliferation policy in writing, formally recognize that U.S. laws supersede any provisions of the pact and acknowledge that the approval of future requests for permission to reprocess U.S.-supplied fuel is not guaranteed.

Glenn's version also would have required presidential certification that all U.S. exporters observe IAEA or equivalent safeguards.

Cranston's language requires the president to certify only that "reciprocal arrangements with China are effective in ensuring peaceful uses" of nuclear technology licensed and resold by China.

"The message this sends to the entire world is that the IAEA system of safeguards is more intrusive than it needs to be," Glenn said.

Cranston said yesterday that his compromise wording will still require the administration to "clarify some loose ends" with China, although it does not require further Chinese action.

Instead, it requires that the president certify to Congress that China has provided "additional information" proving it is not helping to spread nuclear weaponry, that U.S. laws will be observed and that reprocessing approval is not guaranteed.

The resolution also adds that the terms of this pact will not set a precedent for deals with other nations.